Dynamical implications of the orientation of atmospheric eddies: a local energetics perspective

A local quasi-geostrophic energetics analysis indicates that within the jet core, low-frequency (LF) eddies behave baroclinically essentially the same as high-frequency (HF) eddies. They both have a westward tilting vertical structure and both grow baroclinically by transporting heat poleward and by converting eddy potential energy to kinetic energy. However, the difference in the horizontal orientations of HF and LF eddies has several important implications to their amplitude and peak locations, as well as their interaction with stationary waves. The barotropic decay of meridionally elongated HF eddies tends to terminate the growth of HF eddies beyond the jet exit region. The barotropic growth of the zonally elongated LF eddies not only ensure a continuous growth of LF eddies in the jet exit region, but also results in a new baroclinic growth of LF eddies farther downstream due to the presence of the west—east temperature contrast associated with stationary waves. The continuous growth of LF eddies due to both barotropic and baroclinic processes in the jet exit region is consistent with the facts that LF eddies reach maximum variability farther downstream of the two major jet streams and that the LF variability is much stronger than HF eddies. The results of energetics analysis are confirmed by the feedback analysis, showing that HF eddies, being dominated by meridional orientations, mainly act to maintain (damp) stationary waves by locally enhancing (reducing) north—south gradient of the height (temperature) field near the jet core regions. The zonally elongated LF eddies, on the other hand, act to primarily reduce the zonal gradient associated with stationary waves both barotropically and baroclinically.